The present invention relates to a cell phone antenna switching circuit and antenna switching method and, more particularly, to a cell phone antenna switching circuit and switching method which use two built-in antennas in a cell phone which requires continuous transmission/reception as in the CDMA (Code Division Multiple Access) scheme or the like and selectively use the antennas depending on the function of the cell phone which is to be used.
In general, with increasing reductions in the size and thickness of a cell phone, it has excellent portability. The cell phone is placed near the user during speech communication, and hence the influence of the human body on an antenna is a factor that causes a deterioration in reception characteristics and the like.
Of the recent cell phones, cell phones capable of continuous transmission/reception as in the CDMA (Code Division Multiple Access) scheme or the like which realizes good communication quality have been widely used.
Cell phones using the 2-GHz band and frequencies higher than the 2-GHz band, such as cell phones based on the CDMA scheme, allow reductions in antenna length because of short wavelengths they use. For this reason, compact built-in antennas are often applied to such cell phones.
When a built-in antenna is applied to a cell phone, the antenna is mounted in a place where the human body has the smallest influence. In general, since the antenna is greatly influenced by the head of the human body during speech communication, the antenna is often mounted in a place free from the influence of the head during speech communication.
There is a recent trend toward a remarkable increase in the number of users of cell phones who connect to the Internet when executing the mail function using packet communication. That is, some users mainly perform key operation for mail communication with their hands independently of the influences of their heads. Conventional cell phones have been mainly so designed as to avoid the influences of the head of the human body. From now on, cell phones need to be designed in consideration of antenna characteristics (radiation efficiency) when they are mainly used with key operation. In order to realize this, two antennas and a conventional diversity function are used. In this case, the reception electric field levels of the two antennas which have received radio waves are measured, and one of them which has a higher reception electric field level is selected.
In a cell phone designed to perform continuous transmission/reception as in the CDMA scheme or the like, once a selected antenna is determined, the antenna in the use cannot be switched to another one by antenna diversity during continuous transmission/reception in speech communication or packet communication. If an antenna is placed at a portion where the movement of the hand exerts influences, an unfavorable antenna may be erroneously selected during key operation due to unstable reception electric field levels during the key operation. For this reason, demands have arisen for a cell phone having a built-in antenna switching function of automatically selecting an optimal antenna depending on the function.
A conventional cell phone antenna switching circuit is a portable radio apparatus which has two or more antennas capable of transmission/reception and selects a transmission antenna by comparing the reception electric field strengths of the antennas in the diversity reception mode (see, for example, Japanese Patent Laid-Open No. 2002-261665 (pp. 2-4; FIG. 2)).
Some cell phone is designed to obtain good reception characteristics even during speech communication by manually switching built-in antennas even if the cell phone is unfolded into upper and lower housings during speech communication, and the reception characteristics deteriorate when the ear of the user is brought into contact with the speaker in the upper housing and a built-in antenna is located close to the ear (see, for example, Japanese Patent Laid-Open No. 11-177485 (pp. 2-3; FIG. 2)).
In a cell phone designed to perform continuous transmission/reception as in the CDMA scheme or the like, the conventional cell phone antenna switching circuit described above cannot switch an antenna in use to the other antenna by antenna diversity during continuous transmission/reception in speech communication or packet communication after the selected antenna is determined.
In addition, since the cell phone comes into contact with part of the human body during speech communication, the antenna characteristics deteriorate.
If an antenna is placed in a portion where the movement of the hand exerts influences, in particular, since the reception electric field levels become unstable during key operation, an unfavorable antenna may be erroneously selected during the key operation.